Contrasting effects of microbial partners in the rhizosphere:: interactions between Norway Spruce seedlings (Picea abies Karst.), mycorrhiza (Paxillus involutus (Batsch) Fr.) and naked amoebae (protozoa)

The importance of the soil microbial community for plant mineral nutrition and nutrient cycling has long been recognized. One of the most important interactions is the symbiosis of plants with mycorrhizas. In contrast, the effects of soil microfauna on plant performance have so far received little attention, although soil protozoa in particular, have been shown to beneficially affect plant growth. We investigated in a laboratory experiment the impact of mycorrhiza and protozoa and their interaction on plant performance. Spruce seedlings with or without the ectomycorrhizal fungus Paxillus involutus (Batsch) Fr. were grown in microcosm chambers with defaunated forest soil with naked amoebae (Acanthamoeba sp.) or without protozoa for 10 months. The presence of protozoa resulted in the development of a more complex root system by increasing root length (51%), length of fine roots (64%) and number of root tips (43%). The effects of protozoa were more pronounced in the absence of mycorrhiza. In contrast to protozoa, the presence of mycorrhiza resulted in a less complex root system, i.e. root length, length of fine roots and number of root tips were reduced by 47, 47 and 40%, respectively. Shoot height, and stem, shoot and needle mass were at a maximum in the combined treatment with both mycorrhiza and protozoa. The presence of mycorrhiza and protozoa also affected plant nutrient concentrations. In treatments with protozoa shoots of spruce seedlings contained less nitrogen, leading, e.g. to an increased C/N ratio in needles. Conversely, in treatments with mycorrhiza concentrations of phosphorus in needles were increased by a factor of almost two. Mycorrhiza and protozoa also affected rhizosphere microorganisms. Microbial biomass was reduced in the presence of mycorrhiza, mainly due to a reduction in bacterial numbers. Conversely, in the presence of protozoa the length of hyphae in the rhizosphere was reduced. It is concluded that the plant-mycorrhiza mutualism and the bacteria-mediated mutualism between plants and protozoa (microbial loop) complement each other; plant resources presumably are allocated to optimize simultaneous exploitation of both mutualistic relationships. (C) 2001 Elsevier Science B.V. All rights reserved.